In many reservoirs, the original oil-in-place (OIP) is recovered in multiple stages. In an initial stage, usually termed “primary” production, the intrinsic reservoir pressure is sufficient to drive the oil from the subterranean reservoir into the production. Usually, only a fraction of the original OIP is produced by this method—often, up to about 20% of the original OIP is produced. The next stage of production, usually termed “secondary” production, relies on alternative production techniques (other than the intrinsic reservoir pressure) to recover more of the original OIP.
Waterflooding is one type of secondary recovery technique that employs a plurality of wells drilled into the reservoir. The wells may include a plurality of horizontally-spaced vertically oriented wells drilled into the reservoir and/or a plurality of horizontally-spaced horizontally oriented wells drilled into the reservoir. Water is injected under pressure into the reservoir through one or more of the wells, each referred to as an “injection” well. The water increases the reservoir pressure, and as the water moves through the formation, it displaces oil from the pore spaces. The displaced oil is pushed or swept through the formation and into one or more of the other wells, each referred to as a “production” well. The hydrocarbons and any water collected in the production wells are produced to the surface via natural flow or artificial lift (i.e., with or without artificial lift). Waterflooding can be used to recover additional oil, often up to an additional 30% of the original OIP. After this point, the cost of continuing a waterflood often becomes uneconomical relative to the value of the oil produced. Hence, as much as 50% of the original OIP can remain in the reservoir after a reservoir has been extensively waterflooded. In general, waterflooding is used as a recovery technique for light oil (32°-40° API gravity), medium oil (20°-32° API gravity), and some viscous oils such as heavy oil (less than 22° API gravity) and bitumen (less than 10° API gravity).
Thermal recovery techniques are particularly suited for recovering viscous oil such as heavy oil and bitumen. These techniques utilize thermal energy to heat the hydrocarbons, decrease the viscosity of the hydrocarbons, and mobilize the hydrocarbons within the formation, thereby enabling the extraction and production of the hydrocarbons. A steam-assisted gravity drainage (SAGD) operation is one exemplary type of thermal technique for recovering viscous hydrocarbons. SAGD operations typically employ two vertically spaced horizontal wells drilled into the reservoir and located close to the bottom of the reservoir. Steam is injected into the reservoir through the upper, horizontal well, referred to as the “injection” well, to form a “steam chamber” that extends into the reservoir around and above the horizontal injection well. Thermal energy from the steam reduces the viscosity of the viscous hydrocarbons in the reservoir, thereby enhancing the mobility of the hydrocarbons and enabling them to flow downward through the formation under the force of gravity. The mobile hydrocarbons drain into the lower, horizontal well, referred to as the “production” well. The hydrocarbons are collected in the production well and are produced to the surface via natural flow or artificial lift (i.e., with or without artificial lift).
Another thermal technique for recovering viscous hydrocarbons is a “hot” waterflooding operation, also referred to as a hot water injection operation. In a conventional or “cold” waterflood, liquid water is injected into the reservoir without increasing its temperature prior to injection, and thus, is typically injected into the reservoir at a temperature that is less than or equal to the ambient temperature of the reservoir, whereas in a “hot” waterflood, the temperature of the liquid water is increased prior to injection, and thus, is typically injected into the reservoir at a temperature that is greater than the ambient temperature of the reservoir (e.g., the water is heated before being injected into the reservoir). The hot water provides the added benefit of adding thermal energy to the reservoir, which decreases the viscosity of the hydrocarbons, thereby allowing the hydrocarbons to move more easily toward production wells. Accordingly, hot waterfloods are commonly used to recover viscous oils, whereas cold waterfloods are commonly used with light and medium oils.